Reinforcing metal bars (hereinafter “rebar”) are bars, often made of steel, having protruding ribs, which are typically used to reinforce concrete structures. The protruding ribs can take a number of shapes or geometries, including diamond shaped, X-shaped, V-shaped, etc. During the construction of bridges, buildings, and similar structures the rebar is often placed in a concrete form and concrete is poured around the rebar. The ribs in the rebar help to anchor the rebar within the concrete. The rebar adds strength to the structures in which it is used.
In typical rebar manufacturing heated bar stock is fed through rolls to form the cylindrical shaped rebar and protruding ribs. In some applications the ribs on the rebar can be manufactured to form threads that extend around the periphery of the core of the rebar. In such threaded rebar, the external threads are able to receive a nut, collar, coupling, or other apparatus, which has internal threads that engage the external threads on the threaded rebar. Threaded rebar can be used to attach the ends of successive rebar pieces together using a coupling that mates with the threads on the ends of successive pieces of rebar and transfers loads within casted concrete structures, precast concrete structural members, etc. Threaded rebar can also be used to secure metal structures to concrete and rebar foundations (i.e., lampposts, bridges, etc.). Furthermore, threaded rebar can be used as bolts, for example in such applications as rock bolts in mining operations.
Standard rebar and threaded rebar can be manufactured by cold rolling or hot rolling metal billets. In both processes a billet is fed between two cylindrical rolls that form the billet into the rebar. The cylindrical rolls have grooves with notches (i.e. knurls) formed therein to receive a bar and form the core rebar shape and protruding ribs as the bar passes through the rolls. In some rebar manufacturing processes flat dies can replace the cylindrical rolls. The flat dies also have grooves with notches formed therein, and are spaced apart to receive a bar that is rotated between them in order to create threads or ribs along the length of the rebar or a portion thereof.
When threaded rebar is manufactured using cold rolling, the bar is passed through the rolls below the recrystallization temperature of the metal, which increases the strength of the metal, improves the surface finish, and results in tighter tolerances on the rebar core and threaded ribs. However, cold rolling also causes work hardening of the metal, which results in the metal becoming brittle, and thus, more susceptible to cracking at the base of the formed threaded ribs. These problems are particularly acute where threaded rebar is used with a nut or a collar, and in these applications the cold rolled threaded rebar is susceptible to premature thread failure.
In a hot rolling process the bar is passed through the rolls above the recrystallization temperature of the metal, which prevents work hardening that can lead to thread failures. Threaded rebar made from hot rolling results in threaded rebar having uniform tensile strength and elongation characteristics, as well as ribs that are less likely to crack because they are an integral part of the bar and not work hardened. Furthermore, hot rolling allows for the use of steels with higher tensile strength, and hot rolling processes do not require additional bar peeling or swaging of the threaded rebar. The problems with threaded rebar manufactured through hot rolling include the formation of ribs that are coarse and unable to be used in applications requiring tight thread tolerances.
Threaded rebar can also be manufactured by forming standard rebar (utilizing either cold rolling or hot rolling), and thereafter, machining a portion of the rebar to add the desired threads. Machined threads result in tight tolerances; however, machined threads are weaker than cold rolled threads. Moreover, manufacturing threaded rebar by machining the threads significantly increases the manufacturing costs associated with the threaded rebar, as it requires multiple processing steps, as well as time consuming and expensive handling.
There are a number of problems associated with manufacturing threaded rebar using cylindrical rolls in a hot rolling process. Cylindrical rolls are used to form square, cylindrical, or other shaped bars into circular rebar with transverse threads formed into opposite sides of the circular rebar. The transverse threads formed are discontinuous and in some cases not aligned if the cylindrical rolls are not properly synchronized. Moreover, in these processes, two longitudinal ribs are formed along the length of the threaded rebar, which is a result of the excess metal from inconsistencies in the shape of the bar as well as the gap between the cylindrical rolls used to form the threaded rebar. The gap between the rolls is necessary so that the rolls do not rub against each other during the rolling process, since such rubbing may result in frictional heat that could damage the rolling system. The longitudinal ribs that result from processing prevent the threaded rebar from being freely rotatable within a nut or other mating internally threaded coupling. In order to manufacture threaded rebar without longitudinal ribs, additional steps are necessary that machine or shear off the longitudinal ribs. In some processes only the longitudinal ribs are machined off, however, in other processes the entire face of the bar with the longitudinal rib is machined into a flat surface. In still other processes the longitudinal ribs are sheared off using saw-tooth rotary dies, which are spaced apart to shear off sections of the longitudinal ribs located between the transverse ribs on the threaded rebar. In other processes the longitudinal ribs are ground off using a smooth groove rotary die that grinds down the longitudinal ribs. All of these methods present significant drawbacks, including additional processing steps, additional processing time, and additional processing equipment, all of which increase the cost of manufacturing the threaded rebar.
Continuous threaded rebar is more desirable than discontinuous threaded rebar since it increases the tensile strength of the rebar due to the increased surface area contact with the mating nut, threaded bore hole, etc. In some embodiments of the invention, a continuous or significantly continuous transverse rib can be produced through hot or cold rolling processes. However, in order to produce a continuous or significantly continuous spiral transverse rib more than two opposing dies are used (i.e. three or four opposing dies that form the threaded rebar at the same time), whereas in standard rebar manufacturing only two dies are used. The need for more than two dies results in increased equipment costs and increased die set-up costs when changing the tooling between standard rebar manufacturing equipment and continuous or significantly continuous threaded rebar manufacturing equipment. A continuous transverse rib can also be produced on bar stock using processes other than rolling, but these processes are also more time consuming and costly because of the additional equipment costs and tooling set-up times.
Therefore, there is a need to develop methods and systems that can be used to produce threaded rebar at reduced costs and in shorter manufacturing times.